1. Field of the Invention
The present invention relates to novel photoresist compositions and processes for preparing the same utilizing polymers having a low polydispersity via the use of certain chain transfer agents (CTA) with certain monomers to provide said polymers. The polymers incorporating the chain transfer agents can be homopolymers, or made with additional monomers to provide copolymers. These polymers/copolymers are then converted into photoresist compositions for use as such.
2. Background
There is a desire in the industry for higher circuit density in microelectronic devices that are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolutions. The use of shorter wavelength radiation (e.g., deep UV, 190 to 315 nm) than the currently employed mid-UV spectral range (e.g., 350 nm to 450 nm) offers the potential for higher resolution.
Several acid catalyzed chemically amplified resist compositions have been developed for use at short wavelengths, such as those disclosed in U.S. Pat. No. 4,491,628 and Nalamasu et al., “An Overview of Resist Processing for Deep UV Lithography,” 3. Photopolymer Sci. Technol. 4, 299 (1991). The resist compositions generally comprise a photosensitive acid generator and an acid sensitive polymer. The polymer has acid sensitive side chain (pendant) groups that are bonded to the polymer backbone and are reactive towards a proton.
Although chemically amplified resist compositions generally have suitable lithographic sensitivity, in certain applications, their performance can be improved by (i) increasing their thermal stability in terms of thermal decomposition and plastic flow and (ii) increasing their stability in the presence of airborne chemical contaminants.
U.S. Pat. Nos. 4,939,070 and 4,931,379 disclose chemically amplified, acid sensitive resist compositions having increased thermal stability in the post image development stage. Although these resists have suitable thermal stability, they also have lower sensitivity and therefore are unsuitable in certain applications.
MacDonald et al. SPIE 14662 (1991) reported that due to the catalytic nature of the imaging mechanisms, chemically amplified resist systems are sensitive toward minute amounts of airborne chemical contaminants such as basic organic substances. In order to protect the resist from such airborne contaminants, the air surrounding the coated film is carefully filtered to remove such substances. Alternatively, the resist film is overcoated with a protective polymer layer. However, these are cumbersome processes.
U.S. Pat. No. 5,625,020 discloses an acid sensitive, chemically amplified photoresist composition having high thermal stability and stability in the presence of airborne chemical contaminants for use in semiconductor manufacturing. This photosensitive resist composition comprises (i) a photosensitive acid generator and (ii) a polymer comprising hydroxystyrene and acrylate, methacrylate or a mixture of acrylate and methacrylate. The resist has high lithographic sensitivity and high thermal stability. The resist also exhibits surprising stability in the presence of airborne chemical contaminants. However, the disclosed process of preparing the polymer results in poor conversion rates and chemical cleavage of some groups in the repeat units. Thus, there is a need for an improved process for preparing the polymers used in the photoresist compositions.
U.S. Pat. No. 4,898,916 discloses a process for the preparation of poly(vinylphenol) from poly(acetoxystyrene) by acid catalyzed transesterification.
U.S. Pat. No. 5,239,015 discloses a process for preparing low optical density polymers and co-polymers for photoresists and optical applications.
U.S. Pat. No. 5,625,007 discloses a process for making low optical polymers and co-polymers for photoresists and optical applications.
U.S. Pat. No. 5,625,020 discloses a process for making a photoresist composition containing a photosensitive acid generator and a polymer comprising the reaction product of hydroxystyrene with acrylate, methacrylate or a mixture of acrylate and methacrylate.
EP 0813113 Al, Barclay, discloses an aqueous transesterification to deprotect the protected polymer.
WO 94 14858 A discloses polymerizing hydroxystyrene without the protecting group.
WO 98 01478 discloses chain transfer agents used to control the polydispersity of certain polymers.
WO 99 31144 discloses chain transfer agents used to control the polydispersity of certain polymers.
Other patents of interest are U.S. Pat. Nos. 4,679,843; 4,822,862; 4,912,173; 4,962,147; 5,087,772; 5,304,610; 5,789,522; 5,939,511; and 5,945,251.
All of the references described herein are incorporated herein by reference in their entirety.